1. Field of the Invention
The present invention relates to a novel method for injection molding, especially reaction injection molding, articles having an auxiliary material present, at a particular location within the article's bulk material, at a :fixed concentration strip or in a concentration gradient, and to the novel articles so made.
2. The State of the Art
There are a variety of uses for injection molded, polymeric articles. One of the most common types of articles is food packaging, generally made of foamed polystyrene. Because these articles retain liquids, and usually have a surface which is seen by consumers, the injection molding art has devised a number of ways to produce an article having a cover layer and a core; typically the core is foamed and the cover is dense. In the manufacture of articles having a foamed core and a cover layer, two components are injected sequentially into a mold cavity; the first portion comprises the cover layer and the second portion comprises a foamable material to provide the core. Mixing of the materials comprising the first and second portions is detrimental to forming an integral cover layer.
One solution that avoids mixing is described by Garner in GB 1,219,097, by which a third portion of material identical to the cover material of the first portion is injected after the second portion of core material. This order of injection insures that the first portion of material injected for the next article molded is comprised of the cover material, and so the portion of the sprue removed from the article will not contain any portion of foamed material (which would disrupt the integrity of the cover layer). A variation of this method is described by Garner in GB 1,339,445, in which this general concept is applied to a mold having multiple sprue channels. Other methods for solving this type of problem include the simultaneous injection of the cover and core portions using special injection nozzles adapted to extrude the core material physically within the cover material, as described by Hanning in U.S. Pat. No. 3,894,823 and in U.S. Pat. No. 4,014,966 and by Langecker in U.S. Pat. No. 4,035,466, and with molds having movable sections, as described by Barrie in U.S. Pat. No. 4,906,066.
In general, these types of production methods are intended for use with articles formed from two polymeric components which are immiscible, or which are injected or extruded into the mold in the laminar flow regime to avoid mixing. It is known that laminar flow for filling a particular mold geometry in the proper orientation with respect to gravity will result in the earlier injected material residing along the mold surface, corresponding to the surface of the molded article, with the later injected material residing in the interior of the part. See generally C. W. Macosko, RIM Fundamentals of Reaction Injection Molding (New York: Hanser Publishers, dist. by Oxford University Press, 1989), Chpt. 4-6. One example exploiting this ability to inject without mixing is described with respect to the formation of artificial flowers by Kwan, in U.S. Pat. No. 3,221,373, in which a first material of one color is injected into a mold to partially fill the mold, and then a second plastic of a different color is injected so as to travel through the first injected portion and pierce its surface. Another example is given by Schrenk et al., in U.S. Pat. No. 3,716,612, in which two or more thermoplastic materials are injected so as to provide separate phases disposed in a plurality of thin layers wherein each layer corresponds to the particular composition of the respective injected material. Immiscibility may be favored by using fluids of differing viscosities; Kataoka, in U.S. Pat. No. 4,140,672, and Monnet, in U.S. Pat. No. 4,497,763, describe first injecting to partially fill a mold with a fluid having a relatively low viscosity at the molding temperature, and thereafter injecting either a second fluid having a higher viscosity as the core component or a gas to produce a hollow article.
A problem arises in the loss of adhesion between the adjacent layers of immiscible components as the adjacent materials become more dissimilar. Loss of integrity can be diminished by injecting a third, adhesive material coextensively with the first two using various types of nozzle configurations, as described by Monnet in U.S. Pat. No. 3,976,226 and in U.S. Pat. No. 4,052,497.
Other disclosures representative of the foregoing art may be found in the following:
______________________________________ Leggat GB 354,837 Steinmann DE-B 965,442 Unda DE-A 1,015,341 Perisse DE-A 1,177,326 Stacy US 2,418,856 Krystof US 3,513,060 Bodkins US 3,531,553 Boutillier US 3,764,642 Smith US 3,793,415 Lederman et al. US 3,846,223 Garner US 3,873,656 Yasuike et al. US 3,966,372 Hanning US 4,115,491 Hendry US 4,155,969 Hendry US 4,201,742 Olabisi US 4,247,515 Schrenk et al. US 5,202,074 ______________________________________
The foregoing production methods are confined to the production of an article having a single composition or one having multiple layers each consisting of a single composition. The art is typically concerned with articles having a foamed or reinforced core and a good surface appearance provided by a coating layer, essentially a multilayer article consisting of core and coating layers. In practice, it is often difficult to control the thickness of the coating layer by use of the prior art methods. Additionally, a separate coating layer carries a number of disadvantages. As mentioned previously, adhesion between the adjacent layers becomes more problematic as the chemical and physical nature of the immiscible prepolymers and/or polymers diverge. Still further, the integrity of a surface layer exposed to the elements, such as an automobile or spacecraft part, can be compromised by collisions with debris and degradation by exposure to sunlight and temperature variations. A crack in a coating layer jeopardizes the integrity of the entire part. Nevertheless, the art has continued to employ molding techniques which simulate, often to an improved degree, the manufacture of an article by lamination techniques; that is, by molding discrete layers wherein each layer is comprised of a uniform composition throughout, where the outermost layer constitutes a shell or coating layer. These techniques are accompanied by the further problems of intricate injection nozzles and one or more separate coating steps for surface layers.